Rear Differential Fluid change question

My question to all you peeps is, how does one open the fill plug?
What does it look like?
do you need special tool to open this? if not then what tool to use , an allen key etc etc???
I cannot feel any hex profile when I touch the fill plug located on top of the diff assembly, I feel that there is a hole on top of the fill plug but was unable to fit any size allen key in that hole. So how does one open this fill plug???

Any help / guidance would be helpfull.....
.....If any one has a schamatic diagram of what the fill plug looks like even that would help understand wheather mine is stripped or not
???
please help...
Javvy
appreciate the help ............and please I would also vote for this to be put up in the sticky as a DIY as I have seen of late a lot of people asking questions on the Differential service..
Javvy



